1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a trap mounted in the bottom nozzle of a fuel assembly for capturing and retaining debris left in the reactor after assembly, repair and/or replacement operations and thereby preventing entry and lodging thereof in the fuel assembly where the debris can cause cladding perforations in the fuel rods and other damage to the fuel assembly.
2. Description of the Prior Art
During manufacture and subsequent installation and repair of components comprising a nuclear reactor coolant circulation system, diligent effort is made to help assure removal of all debris from the reactor vessel and its associated systems which circulate coolant therethrough under various operating conditions. Although elaborate procedures are carried out to help assure debris removal, experience shows that in spite of the safeguards used to effect such removal, some chips and metal particles still remain hidden in the system.
In particular, fuel assembly damage due to debris trapped at the bottom grid has been noted in several reactors in recent years. The damage consists of fuel rod tube perforations caused by fretting of debris in contact with the exterior of the tube. The debris tends to be relatively thin rectangular pieces, as opposed to pieces which are spherical in shape. Specifically, most of the debris consists of metal turnings which were probably left in the primary system after steam generator repair or replacement. The debris lodges in the region of the lowermost grid within the spaces between its "egg-crate" shaped cell walls and the lower end portions of the fuel rod tubes. Almost all of the debris is deposited just above the four coolant flow openings in the lower core support plate.
Several lower approaches have been proposed and tried for carrying out removal of debris from nuclear reactors. Many of these approaches are discussed in U.S. Pat. No. 4,096,032 to Mayers et al. While all of the approaches described in this patent operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, a need still exists for a fresh approach to the problem of debris removal in nuclear reactors, the new approach must be compatiable with the existing structure and operation of the components of the reactor, be effective throughout the operating cycle of the reactor, and at least provide overall benefits which outweight the costs it adds to the reactor.